1. Field of the Invention
The present invention relates to a method for producing a reinforcing structure, in particular a belt structure for vehicle tyres, comprising the steps of preparing band-like portions each comprising longitudinal threadlike elements arranged parallel to each other and at least partially lined with at least one layer of elastomer material, so as to form at least one belt strip having a continuous circumferential extension about a geometrical axis of rotation.
The invention in question also relates to a tyre for motor vehicles, formed using the abovementioned method.
2. Description of the Related Art
A tyre for vehicle wheels normally comprises a carcass structure essentially composed of one or more carcass plies formed in a substantially toroidal configuration and having their axially opposite side edges engaged with respective annular, circumferentially inextensible, reinforcing structures, usually called “bead wires”. Each annular reinforcing structure is incorporated in a so-called “bead” defined along an internal circumferential edge of the tyre for fixing the latter on a corresponding mounting rim.
The carcass structure has mounted on it, in a radially external position, a belt structure comprising one or more belt strips formed in a closed loop and essentially composed of textile or metal cords incorporated in a layer of elastomer material and suitably oriented with respect to each other and to the cords of the adjacent carcass plies.
A tread band is also mounted in a position radially external to the belt structure, said tread band normally consisting of a strip of elastomer material of suitable thickness. It must be pointed out that, for the purposes of the present description, the term “elastomer material” is to be understood as meaning a composition comprising at least one elastomer polymer and at least one reinforcing filler and if necessary additives of various types (for example, cross-linking agents, plasticizers and other additives known in the art).
A pair of sidewalls is mounted on opposite sides of the tyre, each of them lining a side portion of the tyre lying between a so-called shoulder zone, located in the vicinity of the respective side edge of the tread band, and the respective bead wire.
In the less recently developed manufacturing processes, the formation of each belt strip is performed by applying onto said support device and then cutting to size a continuous strip of rubberized fabric formed by joining, one after another, a plurality of portions obtained from a strip of rubberized fabric including a plurality of longitudinal cords parallel to each other and incorporated in a layer of elastomer material, by cutting said strip at a angle of predefined value with respect to the direction of said cords. More particularly, the operations of cutting and joining on said device are performed so as to obtain a so-called belt ring, in which the cords of each portion are oriented with a predetermined inclination with respect to the circumferential extension of said ring.
The belt structure, which may also comprise two or more strips formed in succession in a radially superimposed arrangement, is associated in a radially external position with the carcass structure, normally in combination with an operating step in which the latter, initially made in the form of a cylindrical sleeve, is formed with a toroidal configuration.
According to the more recent production methods, the carcass ply or plies, as well as each of the belt strips, are obtained by laying a plurality of band-like portions, one after another, in a circumferentially adjacent arrangement, on a toroidal support shaped in accordance with the internal form of the tyre to be obtained. A method of this kind allows elimination or at least limitation of the production of intermediate semifinished products for the purposes of preparation of the tyres and is described, for example, in the European patent application published under No. EP 0 928 680 in the name of the same Applicant.
Moreover, the document WO 99/17920 describes a method and an apparatus which produce a belt strip by means of laying band-like portions cut from a continuous band-like element comprising longitudinal threadlike elements arranged parallel to each other and at least partially lined with at least one layer of elastomer material. Each portion, once cut from the continuous band-like element, is removed by means of gripping members of the magnetic or suction-cup type operated by suitable actuating devices. The gripping members retain the band-like portion at its opposite ends and optionally in a central portion thereof, and are moved so as to bring about the application of said portion onto the external surface of a toroidal support, at a predetermined angle with respect to the circumferential extension of the latter. Once application has been completed, the toroidal support is rotated about its geometrical axis through a predetermined angle, so as to allow the application of a new band-like strip adjacent to that applied previously. The sequential repetition of the steps described above results in the formation of a belt strip extending over the whole circumferential extension of the toroidal support.
The Applicant has ascertained that there exists a series of drawbacks arising from the laying of the band-like portions forming belt strips, as performed in accordance with the teachings of the prior art which result in the formation of belt strips with continuous circumferential edges. In fact, the stepped zone formed by said edges of the belt strips negatively influences the performance of the tyre since it gives rise to a very marked rigidity gradient, i.e. a sudden variation in the rigidity value of the tyre carcass along the edges of the belt structure.
This gradient may produce a “hinge effect” on the edge of the tyre, i.e. the generation of a preferential line of deformation in the tyre structure, along which the flexing stresses of the tread are concentrated, with the consequent greater wear of the latter, in particular on the shoulders. This effect is particularly significant when the stepped zone is located at the bottom of the lateral cavity of the tread, where, in addition to the greater deformability of the structure, there is also a reduced thickness of the rubber forming the tread band. It is pointed out that this problem has become more acute with the spread of tyres with a radial carcass since, in the former tyres with a cross-ply carcass, the carcass had at least one pair of radially superimposed plies and the belt structure, when present, usually consisted of belt strips reinforced with textile cords, so that any rigidity gradient was very much less marked, if not negligible.
Moreover, since the transverse cut for obtaining the portions of fabric for the belt strips interrupts the longitudinal metal reinforcing elements, the ends of these threadlike elements in the belt structure thus obtained are truncated and devoid of protective lining, being aligned substantially along the circumferential edges of the structure. This circumferential alignment results in a preferential path along which, during use of the tyre, separations and tears between rubber and metal—due to the low degree of adhesion between rubber and metal at the ends of the interrupted metal elements—may spread, with the further possibility, in the event of water infiltration, of corrosion phenomena which then rapidly spread along the surface of said threadlike elements.